High Strength Steel Sheet Hardness Gradient Control
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Solution Overview
Problem
Existing high strength steel sheets face challenges in achieving both excellent bendability and bending load due to variations in hardness and the lack of control over the hardness gradient in the transition zone between the soft surface layer and the hard inner layer, which can lead to insufficient performance in automotive applications.
Innovation Solution
The development of a high strength steel sheet with a soft surface layer and a hard inner layer, where the average hardness of the soft layer is controlled, and the hardness gradient in the transition zone is minimized, along with the inclusion of retained austenite in the middle part to enhance ductility, using specific production methods such as hot rolling and annealing to achieve optimal microstructural properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a soft layer is provided at the surface layer to improve bendability, then the tensile stress and compressive stress at the surface are eased and bendability is improved, but the bending load deteriorates
Solution Approach 1:
The patent applies local quality by creating distinct layers with different hardness characteristics: a soft surface layer (5-20 μm depth with Vickers hardness 150-300) to improve bendability, a transition zone with controlled hardness gradient to prevent stress concentration, and a hard inner layer (Vickers hardness 400-600) to maintain bending load. This spatial differentiation of material properties resolves the contradiction between surface softness for bending and overall hardness for load bearing.
Solution Approach 2:
The patent employs parameter changes by precisely controlling the depth, hardness, and hardness gradient of the surface layer through specific production parameters including cooling rates (20-100°C/s), finishing temperatures (700-950°C), and chemical composition (C: 0.20-0.40%, Si: 0.80-1.50%, Mn: 1.50-3.00%). These parameter adjustments optimize both bendability and bending load performance.
2Ease of operation
If the average hardness of the soft layer is lowered and the thickness is increased to improve bendability, then bendability is improved, but the bending load deteriorates
Solution Approach 1:
The patent maintains optimal bendability while preserving bending load by defining specific local quality parameters: the soft surface layer is limited to 5-20 μm depth with Vickers hardness of 150-300, preventing excessive softening that would reduce bending load. The transition zone (20-50 μm depth) provides a controlled hardness gradient, and the hard inner layer (Vickers hardness 400-600) ensures sufficient bending load capacity.
Solution Approach 2:
The patent optimizes the competing parameters of surface layer thickness and hardness through controlled parameter changes during production: cooling rates of 20-100°C/s and finishing temperatures of 700-950°C produce the desired 5-20 μm soft layer with appropriate hardness, while the chemical composition (C: 0.20-0.40%, Si: 0.80-1.50%, Mn: 1.50-3.00%) ensures the hard inner layer achieves Vickers hardness of 400-600 for sufficient bending load.
3Ease of operation
If numerous methods are used to soften the surface layer to improve bendability, then bendability may be improved, but the bending load remarkably deteriorates
Solution Approach 1:
The patent extracts and isolates the essential function of surface softening to a single controlled mechanism: controlled rolling with specific parameters (cooling rates of 20-100°C/s, finishing temperatures of 700-950°C) that produce a uniform soft surface layer. This eliminates the need for multiple softening methods that would create heterogeneous microstructures and reduce bending load, while still achieving excellent bendability.
Solution Approach 2:
The patent achieves optimal bendability while maintaining bending load through precise parameter changes in a single softening mechanism: controlling cooling rates (20-100°C/s), finishing temperatures (700-950°C), and chemical composition (C: 0.20-0.40%, Si: 0.80-1.50%, Mn: 1.50-3.00%). This controlled approach creates a uniform soft surface layer with appropriate hardness gradient, avoiding the bending load deterioration associated with multiple uncontrolled softening methods.
4Ease of manufacture
If the hardness gradient at the transition zone between soft layer and hard layer is not controlled, then production is simpler, but the bendability is insufficient due to stress concentration
Solution Approach 1:
The patent controls the hardness gradient in the transition zone through parameter changes in the rolling process: cooling rates of 20-100°C/s and finishing temperatures of 700-950°C produce a gradual hardness transition from the soft surface layer to the hard inner layer. This controlled gradient prevents stress concentration at the interface, improving bendability while maintaining production simplicity through a single integrated rolling process.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting steel sheet exhibits improved bendability and ductility, making it suitable for automotive parts with enhanced tensile strength and reduced risk of fracture, while maintaining a high bending load capacity.
Implementation Method 1
using specific production methods such as hot rolling and annealing to achieve optimal microstructural properties
Implementation Method 2
using specific production methods such as hot rolling and annealing to achieve optimal microstructural properties
Data Source
Figure 1
Figure 2~3
AI summary
High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 µm and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of more than 0.60 time and 0.90 time or less the average Vickers hardness of the sheet thickness 1/2 position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.